Cationic electrodeposition coating compositions usually contain a resin emulsion and a pigment dispersion paste. In such a cationic electrodeposition coating composition, an organic tin compound has been widely used as the curing catalyst so far. However, the organic tin compound may probably be limited in its use by a trend to environmental regulation. It is therefore necessary to develop a catalyst as a substitute for the organic tin compound.
Studies are being made that use a bismuth compound as the curing catalyst for the cationic electrodeposition coating composition. However, the catalyst activity of a bismuth compound such as bismuth oxide or bismuth hydroxide is not adequate and therefore the resulting coating film cannot be sufficiently cured only by simply dispersing the bismuth compound in a pigment dispersion paste. This also poses a problem that addition of the bismuth compound in a cationic electrodeposition coating composition deteriorates a storage stability of a coating composition or pigment dispersion paste, causing coagulation during storage.
A method is disclosed in which after a bismuth compound is mixed and dissolved in advance in an amine-containing carboxylic acid such as an amino acid, the obtained mixture is used to prepare a pigment dispersion paste (Patent Document 1: Japanese Patent No. 3293633). Also, a method is disclosed in which after a bismuth compound is mixed and dissolved in advance in lactic acid, the obtained mixture is added to a coating material (Patent Document 2: Japanese Patent No. 3874386). As described in these documents, these methods each have the advantage that the activity of the catalyst is improved by dissolving bismuth in advance. In these methods, on the other hand, it is necessary to use a large amount of acid to dissolve bismuth in advance. The use of a large amount of acid to dissolve bismuth also gives rise to problems concerning increase in the electroconductivity of an electrodeposition coating composition, deterioration in electrodeposition coating workability, and deterioration in the appearance of the obtained coating film.